Scintillating materials, especially in the form of single crystals, are widely used to detect ionizing radiation both in the form of electromagnetic waves of low energies and gamma-rays, X-rays, cosmic rays and particles. The scintillation mechanism relies on converting the energy of the incoming photons or particles into light which is within or reasonably close to the visible range, so it can be detected by standard photo-detectors. The key characteristics of crystal scintillators are their ability to absorb X-rays or gamma-rays, which to a first approximation is a function of ρ·Z4 (where ρ is the density and Z is the effective atomic number), scintillation light yield, scintillation decay time and energy resolution. The shorter scintillation decay time the better time resolution of a detector. The smaller the numerical value of energy resolution the better the quality of a detector. Even an energy resolution of about 7% (a standard value for NaI) would suffice to obtain good results in solving certain types of problems. Of particular interest are monocrystalline scintillator types (scintillators in the form of single crystals). In order to be used efficiently such scintillators should be nontoxic, hard enough, and (which is very important) they should be nonhygroscopic.
Most known and most commonly used are scintillator crystals of sodium iodide type doped with thallium NaI(Tl), discovered by Robert Hofstadter in 1948. These materials offer high light yields of about 38000-40000 photons/Mev, they have been the basis of modern scintillators ever since and remain predominant in this field of application. Materials like NaI(Tl) are characterized by medium energy resolutions (about 7% at 662 key 137Cs), but they have high fluorescence decay time constant, that is equal to about 230 ns. Similarly, CsI(Tl) has a high decay time constant, it is longer than 500 ns. Scintillating materials of the NaI(Tl) type are highly hygroscopic, which is a decided practical disadvantage. Due to above reasons the development of new scintillating materials for improved performance still remains the topic of a large body of research.
In 2001 publications appeared (see below) about a new group of scintillators based on lanthanum halides doped with cerium, including the ones based on lanthanum chloride LaCl3 and lanthanum bromide LaBr3. These scintillators have decay time constants of about 20 ns and light yields comparable to those in materials of the NaI(Tl) type and even higher.
There are rare earth element-based scintillating materials like Ln1-xCexCl3 and Ln1-xCexBr3, where Ln is chosen from lanthanoids or mixtures of lanthanoids and x is the molar level of substitution of Ln by cerium and there are radiation detectors using such scintillating materials (Patent applications PCT/EP01/01837 and PCT/EP01/01838 “Scintillator crystals, a process to manufacture them, the use of these crystals”. Publications PCT WO 01/60944 of 23 Aug. 2001 and WO 01/60945 of 23 Aug. 2001). Specifically, single crystals of LaCl3:Ce and LaBr3:Ce offer short scintillation decay times with a fast scintillation component which are equal to 25-36 ns and an excellent energy resolution of about 2.9%-3.1%. However, in spite of all of the above virtues these scintillators have a decided disadvantage of being highly hygroscopic. Under atmospheric conditions the use of such crystals in radiation detectors without special moisture protection is highly problematic.
There is an inorganic scintillating material also available in the form of a single crystal which contains praseodymium halide and cerium halide and is of a general formula Pr(1-x-y)LnyCexX3, where Ln is chosen from the following group of elements: La, Nd, Pm, Sm, Eu, Gd, Y or their mixtures, X is chosen from the group of elements: Cl, Br, I or their mixtures, x is the molar level of substitution of praseodymium (Pr) by cerium, y is the molar level of substitution of praseodymium by lanthanum (Patent application PCT EP2006/066427). High Light Yield Fast Scintillator. Publication of the application PCT WO 2007/031583 of 22 Mar. 2007). The patent application also describes radiation detectors based on the said scintillating material. The scintillating material of the above invention, which is of interest for gamma radiation detection applications, is inferior to LaBr3:Ce as claimed in WO 01/60945 with respect to energy resolution, but it is superior to LaBr3:Ce with respect to count rate (more than 100 kcps (kilocounts per second) or even above 1 Mcps (megacounts per second)). Such material is of particular interest for high count rate detector applications, especially in PET (positron emission tomography) scanners. A disadvantage of the said material is that its energy resolution needs to be improved by producing material with good crystallinity and homogeneity using well controlled furnaces and an adequate choice of thermal conditions, of thermal gradients at the solid/liquid interface. The above materials are highly hygroscopic.
There are scintillation crystals of formula Ln(1-y)CeyX3:M, where Ln(1-y)CeyX3 is the chemical composition of the matrix material, Ln is one or more elements chosen from the group of rare earth elements, X is one or more elements from the group of halogens, M is a trace impurity element the matrix material is doped with, which can be one or more elements chosen from the group: Li, Na, K, Rb, Cs, Al, Zn, Ga, Be, Mg, Ca, Sr, Ba, Sc, Ge, Ti, V, Cu, Nb, Cr, Mn, Fe, Co, Ni, Mo, Ru, Rh, Pb, Ag, Cd, In, Sn, Sb, Ta, W, Re, Os, Ir, Pt, Au, Hg (US Patent application No2008/0067391, publication 2008 Mar. 20). The introduction of the above elements into the crystal matrix makes it possible to obtain crystals whose peak of scintillation intensity is shifted to longer wavelengths, which improves the operating efficiency of detectors based on such crystals and commonly used together with photomultipliers with a bialkali photocathode as the photo-detector. The chief drawback of the proposed scintillation crystals is their high hygroscopicity.